Geopositioning techniques for locations with obscured satellite signals

ABSTRACT

In a system for geopositioning receivers in areas substantially inaccessible to satellite signals, multiple access points configured to periodically transmit management frames (i) via a single shared communication channel and (ii) using a modulation scheme associated with a rate of at least 50 Mbps. A database stores respective locations for each of the access points. A portable computing device is configured to retrieve the location of the access points from the database and, when moving through a region in which the of access points are disposed, receive at least one management frame from each of the access points within a limited time interval on the single shared communication channel. The portable computing device is further configured to determine a current position of the portable computing device based on the management frames using the retrieved locations of the access points.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims the benefit of U.S. Provisional PatentApplication No. 62/286,804, filed Jan. 25, 2016 and titled“GEOPOSITIONING TECHNIQUES FOR LOCATIONS WITH OBSCURED SATELLITESIGNALS,” and U.S. patent application Ser. No. 15/211,999, filed Jul.15, 2016 and titled “AUTOMATICALLY DETERMINING LOCATIONS OF SIGNALSOURCES IN AREAS WITH LIMITED SATELLITE COVERAGE,” the disclosures ofwhich are incorporated herein by reference in their entirety for allpurposes.

FIELD OF THE DISCLOSURE

The present disclosure relates to automatic provisioning of wirelesssignal sources for subsequent use in geopositioning in areas with poorsatellite coverage.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentdisclosure.

Navigation software available today cannot provide precise reliablenavigation inside tunnels. More generally, geopositioning systems thatrely on signals from satellites, such Global Positioning Service (GPS),do not work in areas where these signals are completely or substantiallyoccluded by obstacles. Some devices in these situations rely on deadreckoning, or estimation of a change in position based on the speed atwhich the device is moving. However, dead reckoning accumulates errors,and accuracy quickly diminishes with changes in speed.

Meanwhile, navigation in tunnels remains a relevant problem as driversoften slow down and sometimes even come to a complete stop when checkingfor underground exits or turns. Especially as more and more driverstoday become dependent on navigation systems telling them where and whenthey should turn, reliable geopositioning everywhere along a route,including inside tunnels, is important.

To use low-energy signal sources such as Bluetooth® or Wifi™ “beacons,”or Wifi™ access points, for geopositioning in areas with limitedsatellite coverage, the locations of the signal sources must beestablished prior to geopositioning receivers. Determining locations ofsignal sources can require land surveying, precise measurements, preciseinstallation, etc. For example, to install signal sources at preciselyknown locations (or precisely determining the locations immediately uponinstallation) in general is difficult, time-consuming, and potentiallydisruptive of the operation of the tunnel.

SUMMARY

A system of this disclosure in general allows a receiver moving througha geographic area with poor or no satellite coverage to quickly andreliably determine its geographic position based on signals receivedfrom access points. To this end, the system reduces the time it takesthe receiver to collect management frames from multiple access points togenerate a positioning fix (using triangulation/trilateration, forexample). More particularly, the access points operating in the systemare configured to broadcast management frames such as beacons on thesame channel. To address problems associated with overcrowding thechannel with a large number of access points, the system reduces thetime it takes to generate a beacon and receive a response by changingthe data rate, for example.

One particular embodiment of these techniques is system forgeopositioning receivers in areas substantially inaccessible tosatellite signals. The system includes multiple access points configuredto periodically transmit management frames (i) via a single sharedcommunication channel and (ii) using a modulation scheme associated witha rate of at least 50 Mbps. The system further includes a database thatstores respective locations for each of the access points. Stillfurther, the system includes a portable computing device configured toretrieve the location of the access points from the database and, whenmoving through a region in which the of access points are disposed,receive at least one management frame from each of the access pointswithin a limited time interval on the single shared communicationchannel. The portable computing device is configured to determine acurrent position of the portable computing device based on themanagement frames using the retrieved locations of the access points.

Another embodiment of these techniques is a method for geopositioningreceivers in areas with limited satellite signal coverage. The methodincludes configuring, by one or more processors, access points toperiodically transmit management frames (i) via a single sharedcommunication channel and (ii) using a modulation scheme associated witha rate of at least 50 Mbps. The method also includes providing, by oneor more processors, respective locations for each of the plurality ofaccess points to a portable computing device, so that the portablecomputing device, when moving through a region in which the accesspoints are disposed, is configured to receive at least one managementframe from each of the access points within a limited time interval onthe single shared communication channel and determine a current positionof the portable computing device based on the management frames usingthe retrieved locations of the access points.

Yet another embodiment is a method for automatically provisioning accesspoints for operating in a geopositioning system. The method includesobtaining an indication of one-dimensional geometry along which accesspoints are disposed, where the one-dimensional geometry describes aregion of physical space. The method further includes receiving, fromthe access points, signal data indicative of distances between at leastseveral of the access points, where the signal data is generated by theaccess points transmitting management frames. The method also includesdetermining, using the indication of the one-dimensional geometry andthe received signal data, positions of the access points along theone-dimensional geometry, and using the determined positions of theaccess points to geoposition portable computing devices travellingthrough the region physical space.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an example environment in which a mobile devicemoving through an area of limited satellite signal coverage determinesits current location using signals from several signal sourcesconfigured using the techniques of this disclosure;

FIG. 2 is a block diagram of an example computing system that can beimplemented in the environment of FIG. 1;

FIG. 3 is a flow diagram of an example method for geopositioning amobile device that can operate in the system of FIG. 2;

FIG. 4 is a flow diagram of an example method for configuring signalsources that can operate in the system of FIG. 2; and

FIG. 5 is a diagram that schematically illustrates signal sourcesstatically arranged along a path that traverses an area with limitedsatellite coverage, the locations of which the system of FIG. 2 canautomatically determine;

FIG. 6 is a graph of signal strength as a function of time, as measuredby a receiver moving past statically arranged signal sources, such asthose illustrated in FIG. 5;

FIG. 7 schematically illustrates extrapolation of signal data toautomatically determine a time when a signal source likely was obscuredby the roof of the vehicle in which the receiver was travelling, whichcan be implemented in the system of FIG. 2;

FIG. 8 is a simplified diagram of changes in signal strength over timeas measured by a vehicle moving past signal sources at different speeds;

FIG. 9 schematically illustrates automatic correction of estimates ofsignal source location in view of vehicle speed, which can beimplemented in the system of FIG. 2;

FIG. 10 is a flow diagram of an example method for determininggeographic positions of signal sources in areas with limited satellitecoverage, which can be implemented in the system of FIG. 2;

FIG. 11 is a diagram that schematically illustrates automaticdetermination of signal source locations using RTT measurements, whichcan be implemented in the system of FIG. 2; and

FIG. 12 is a flow diagram for provisioning of the access points that canoperate in the system of FIG. 2.

DETAILED DESCRIPTION

As briefly outlined above, a system that implements the techniques ofthis disclosure automatically determines how proximity beacons or othersignal sources are arranged along a path through an area with limitedsatellite coverage, using signal data collected by one or multiplereceivers (such as smartphones) moving through the area. The system alsocan utilize other input to makes the determination more accurate,depending on the implementation. Once the locations of the beacons havebeen determined, the system can geoposition receivers subsequentlymoving the area along the path, or the beacons can transmit thedetermined location information in management frames and/oradvertisement transmissions to allow receivers to geoposition themselvesautonomously.

The techniques of this disclosure can reduce the amount of time requiredto provision beacons or other signal sources for geopositioning as wellas eliminate the need for operators to obtain and manually enterlocation information into the beacons and/or a centralized database.

An example environment in which these techniques can be used and asystem that can operate in this environment are discussed first withreference to FIGS. 1 and 2. For further clarity, example geopositioningtechniques using signal sources operating in this environment arediscussed with reference to the flow diagrams of FIGS. 3 and 4. In someembodiments, the system of FIG. 3 uses these geopositioning techniquesafter the locations of the signal sources are automatically determinedusing one or several of the approaches discussed with reference to FIGS.5-12.

Example Environment and System Components

FIG. 1 schematically illustrates an example environment 10 in which avehicle 12 moves through a region in which little or no satellite signalcoverage is available due to obstacles 14 that substantially block radiosignals. For example, the obstacles 14 can the walls and the ceiling ofa tunnel. Signal sources 20 are disposed along the direction in whichvehicles move through the tunnel and are configured to transmitmanagement frames such as beacons. The signal sources 20 can beproximity beacons that operate according to Bluetooth® or Wifi™standards, for example, or wireless access points via which devices canaccess a communication network. The signal sources 20 may include one ormore processors and a non-transitory memory storing instructions thatcan be executed on the one or more processors. The vehicle 12 isequipped with a wireless receiver or transceiver 22 that operates as awireless client or “station” when interacting with the signal sources20. None of the components in FIG. 1 or distances between thesecomponents are drawn to scale.

In operation, the signal sources 20 transmit management frames in amanner that, on the one hand, allows multiple signal sources to co-existon the same channel and, on the other hand, allows the station 22 toreceive management frames from several different access pointssubstantially at the same time, i.e., within a time interval duringwhich the vehicle 12, even when moving at 100 km/h, travels no more thanseveral meters. The term “channel” in this disclosure should beunderstood to include a frequency/frequency band, or another suitableform of partitioning a radio resource for concurrent use by multipletransmitters. As discussed in more detail below, the signal sources 20to this end are configured to transmit management frames on a samechannel using a suitable modulation scheme.

The station 22 can be a navigation system built into the vehicle 12, aportable navigation device, a smartphone with navigation software, awearable device such as a smartwatch, etc. Further, although FIG. 1illustrates a personal vehicle, in other environments the station 22 canoperate in trains, busses, and other types of vehicles.

The station 22 can be implemented in a mobile device 100 illustrated inFIG. 2. The mobile device 100 can include one or more processor(s) 102such as CPUs, for example, a wireless interface 104 such as a wirelesscard that supports communications using communication protocols thatconform to such standards as IEEE 802.11 (Wi-Fi) and/or 802.15(Bluetooth), for example, a user interface 106 such as touchscreen, aspeaker, etc., and a non-transitory computer-readable memory 108, whichcan include persistent and/or non-persistent storage components.

In an example implementation, the memory 108 stores a software platform120, which can include an operating system (e.g., Android®, iOS®),additional libraries, plug-ins, various porting layers, etc.Applications 124 can access various functions of the platform 120 via anAPI layer 122. For example, the platform 120 can include functionalityfor determining the location of the mobile device 100 using availablesignals, such as GPS signals, Wi-Fi signals, etc., and exposed some ofthe results of geopositioning via an API. More specific examples of suchfunctionality include Fused Location Provider (FLP) available onAndroid® operating system and Location Services (LS) available on iOS®.

With continued reference to FIG. 2, the mobile device 100 normally canaccess a network server 130 via a wireless network 132. The networkserver 130 can retrieve data related to the signal sources 20 from anaccess point database 134 and provide this data to the mobile device100. The access point database 134 can be implemented in one or moreservers in any suitable manner. The data can include identifiers ofaccess points, channel selection, geographic locations of the accesspoints, etc. Using this data and signal strength measurements (or othersuitable measurements) of management frames from the signal sources 20,the mobile device 100 can determines its current location. When in atunnel, the mobile device 100 may not be able to access the server 130,and in some implementations may rely on pre-cached data related to thesignal sources 20. A signal source configuration system 140 canconfigure the signal sources 20 as discussed below and populate thecorresponding data records in the database 134.

As also discussed below, one or more receivers such as smartphones canparticipate in collection of signal data from the signal sources 20A-Cand providing the signal data to the source configuration system 140,which then automatically determines the locations of the signal sources20A-C using this data. In some implementations, the user of a smartphonethat participates in the determination of signal source locationsoperates certain controls and/or installs certain applications to allowthe smartphone to report the measurements to the signal sourceconfiguration system 140.

Geopositioning Using Access Point Signals

Examples of functionality for determining the location of the mobiledevice 100 using available signals include Fused Location Provider (FLP)available on Android® operating system and Location Services (LS)available on iOS®. Both FLP and LS can activate Wi-Fi navigation inresponse to determining that the GPS signal is not available. However,the existing platforms generally provide only limited access to signaldata and allow little, if any control, over processing signals from thesignal sources 20. In particular, the existing platforms do not providecontrol to applications (such as the applications 124) over the order inwhich wireless channels are scanned, nor do these platforms provide anindication of the scan order or control over scanned channels.

On the platform 120 and other platforms available today, the scanninginterval is approximately five seconds, with each scan taking up to 2.5seconds. During a scan, the platform 120 iterates through a set ofchannels (e.g., 13 on a Wi-Fi network) to detect management frames andcalculates Received Signal Strength Indication (RSSI) or anotherquantitative metric of signal quality for management frames from thesignal sources 20A. When the platform receives the first managementframe from the signal source 20A at the beginning of the 2.5-second scanon the first channel and receives the last management frame from thesignal source 20B at the end of the 2.5-second scan on the last channel,the mobile device 100 can be displaced by a significant distance duringthis interval. For example, if the mobile device 100 operates in avehicle travelling at 100 km/h, the first management frame and the lastframe can be received and measured at locations that are approximately70 meters apart.

As a result, when one of the applications 124 initiates a locationrequest 150 via the API layer 122, the platform 120 can provide alocation estimate 152 in which possible displacement of the mobiledevice during a scan introduces a significant error.

In one possible embodiment, the platform 120 exposes the scan order viathe API layer 122, so that an application can retrieve an estimate ofthe speed of the mobile device 100 during a scan (using dead reckoningtechniques, for example), determine when exactly management frames fromparticular access points were received within the scan period, andattempt to mitigate the error in view of the speed. Alternatively, errormitigation using dead reckoning could be implemented in the platform120, so that the accuracy of the location estimate 152 returned via theAPI layer 122 is improved.

However, as discussed above, dead reckoning techniques are subject toerror accumulation. Moreover, even if a certain platform is modified toprovide an indication of scan order or exposes additional controls forcontrolling the scan order, it is desirable to have geopositioningsolutions that do not depend on specific types or version of platformsoftware.

To provide a solution that does not require installing specializedsoftware on or otherwise reconfiguring the mobile device 100, the signalsource configuration system 140 configures the signal sources 20A, 20B,. . . 20N to transmit Wi-Fi management frames on the same singlechannel. As a result, the platform 20 receives the Wi-Fi managementframes from the signal sources 20A, 20B, . . . 20N and calculates theRSSI for these frames at approximately the same time, i.e., during thescan of the same channel during a multi-channel scan procedure. Eventhough the time the Wi-Fi management frames are received and measuredand the time of the scan reported by the platform 120 can be separatedby as much as 2.5 seconds, the overall precision of geopositioningincreases because the measurements from multiple access points arecollected at approximately the same time.

Further, the signal source configuration system 140 modifies the rate atwhich the signal sources 20 transmit management frames to addresspotential overcrowding of the shared channel. According to the existingstandard, management frames are transmitted using Direct Sequence (DS)modulation, at 1 mbps. As a result, it takes approximately 2.5 ms tosend each beacon, or 25 ms out of each second of required air time forevery access point. This correspond to 1/40 the total air time, whichmakes the spectrum very crowded once multiple access points are added tothe system 10. Both management frames and beacons adhere to the CSMAprotocol. Assuming for example 42 access points transmitting, with 20being “visible,” the chances of finding an available timeslot are verysmall.

The avoid overcrowding for the reasons discussed above, the signalsource configuration system 140 according to one example implementationconfigures the signal sources 20 to transmit management frames usingOrthogonal Frequency Division Multiplexing (OFDM), so as to reach thespeed of 54 mpbs. Experiments with 42 access points have shown that theair time required for each beacon/probe response was reduced toapproximately 0.15 ms. The approximate probability of finding anavailable transmission slot, assuming all 42 access points are“visible,” is approximately 93%.

Referring to FIGS. 1 and 2, experiments have been conducted to place theaccess points similar to the signal sources 20 in a tunnel, once every40 m. The access points were installed on the side of the tunnel at theheight of approximately 4 m. TL-WR740N devices, with the Atheros AR9xchipset, were used as access points. The access points were configuredusing OpenWRT firmware (it is noted that OpenWRT can be used with avariety of suitable access points, from various manufacturers). Theexperiments have shown that, even with multipath and poor line-of-sightfactors, a receiver “sees” between −46 and −66 dBm when positionedbetween two adjacent access points, and between −32 and −52 dBm when atthe shortest distance from one of the access points (with the other twoaccess points corresponding to the signal strength of approximately −52to −72 dBm, when the transmit power is set to +20 dBm. By simplyaveraging the RSSI, the geopositioning error was reduced to less than 16m. Moreover, with Bluetooth transmitters, the error was reduced to lessthan 10 m,

Thus, access points of this disclosure are placed in a tunnel andconfigured to transmit beacons or other management frames in a mannerthat allows a mobile device moving through the tunnel to determine itsgeographic position with sufficient accuracy. As discussed above, accesspoints are configured to (i) transmit management frames on the samecommunication channel so as to eliminate the need for the mobile deviceto switch between channels when scanning for management frames, and (ii)transmit management frames using a modulation scheme (e.g., OFDM) thatallows all the management frames to be collected within a short timeinterval (e.g., 15 ms) within which the vehicle has not moved far enoughto significantly impact positioning accuracy.

Example Geopositioning Methods

For further clarity, FIG. 3 depicts a flow diagram of an example method200 for geopositioning a mobile device. The method can be implemented inthe mobile device 100 of FIG. 2 or the station 22 of FIG. 1, forexample. Although the method 200 is discussed with reference to accesspoints, it will be understood that these techniques can be used with anysuitable signal sources, as indicated above.

The method 200 begins at block 202, where locations of a set of accesspoints are retrieved. The access points may define a set sufficient forgeopositioning a mobile device in a tunnel, for example. Depending onthe implementation, the locations of the access points can be retrievedprior to the mobile device entering the tunnel or as the mobile deviceis travelling through the tunnel when cellular coverage is available,for example.

At block 204, management frames are received at the mobile device frommultiple access points on the same channel during a single scan, and theRSSI of the management frames (or another suitable metric) is calculatedat block 206. The mobile device then can determine its geographicposition using the RSS values of the management frames and thepreviously received locations of the access points.

FIG. 4 depicts a flow diagram of an example method 200 for configuringaccess points. The method 200 can be implemented in the signal sourceconfiguration system 140, for example.

At block 302, multiple access points are placed in an area with occludedsatellite signals, such as along a wall of a tunnel. The access pointsthen are configured to transmit management frames on the same channel(block 304). At block 306, the access points are configured to transmitat a relatively high modulation rate, such as 54 Mbps, using OFDM forexample.

Automatically Determining Locations of Signal Sources

The diagram 350 of FIG. 5 schematically illustrates several signalsources 360A-G statically arranged along a path 352, which can be a roadthrough a tunnel or another area with limited satellite coverage, suchas a lower-level road occluded from satellites by another level of theroad. The signal sources 360A-G may be arranged at least approximatelyat distance d from each other, with the separation between pairs ofadjacent signal sources varying depending on the particular setup. Inthose configurations where the separation between some of the adjacentsignal sources significantly deviates from distance d, the signal sourceconfiguration system 140 can use certain techniques to determinedistances d′, d″, etc., as discussed below.

As a more specific example, a technician can affix proximity beacons tothe ceiling of the tunnel, trying to maintain the separation of distanced between the beacons. When the technician is known to use relativelyprecise spacing techniques, the signal source configuration system 140can be configured to implement the assumption that each pair of adjacentsignal sources 360A-G is separated by distance d. Moreover, the signalsource configuration system 140 can be configured to implement thisassumption only for the purposes of generating an initial estimate ofwhere along the path 352 the signal sources 360A-G are located. Thesignal source configuration system 140 then can adjust the initialestimate once additional or improved data becomes available.

In any case, the signal source configuration system 140 initially maynot have any information about the location of the signal sources360B-360F. Regarding the signal sources 360A and 360G disposed at therespective ends of the path 352, these devices in some scenarios may bedisposed near the two entrances into the tunnel. In these cases, thelocation of the signal sources 360A and 360G may be determinedrelatively easily using respective GPS positioning fixes, for example.

A vehicle 370 can be similar to the vehicle 12 of FIG. 1 include areceiver such as the station 22, for example. The receiver in generalcan be any suitable device capable of detecting, measuring the strengthof, and at least partially processing wireless signals from the signalsources 360A-G. The vehicle 370 in the example scenario illustrated inFIG. 5 moves along the path 352 in the direction 380, and the receiverof the vehicle 370 collects signal data that indicates variations in thestrength of the corresponding signals from the signal sources 360A-G, asa function of time. The receiver then may provide the signal data to thesignal source configuration system 140, which in turn may use thissignal data to first determine the relative order of the signal sources360A-G.

As a more specific example, FIG. 6 illustrates signal data which thereceiver in the vehicle 370 may collect while traveling along the path352 in the direction 380. Each of the seven illustrated signals caninclude an identifier of the corresponding signal source in the form ofany suitable combination of numbers and/or characters. Peaks 400A-Gcorrespond to the maximum strength (e.g., RSSI) the signals fromdifferent signal sources reach at the corresponding times. For example,the peak 400A occurs at time t₁, when the receiver mounted in thevehicle 370 detects the strongest signal from the source signal 360A,which typically occurs when the vehicle 370 is in closest proximity tothe signal source 360A (except for the situations discussed withreference to FIG. 7). At this time, the signals from the other signalsources 360B-G are weak or not practically detectable at all.

At time t₂, the receiver mounted in the vehicle 370 detects thestrongest signal from the source signal 360B (peak 400B), while thesignal from the source signal 360A drops by about 20 dB. At this time,the vehicle 370 is likely in close proximity to the signal source 360B.A similar pattern can be observed for the other signals: the receiverobserves a peak value of signal strength for one of the signal sourcesand low signal strength for the remaining signal sources. Before thereceiver reaches a certain signal source, the signal strength for thesignal source gradually increases but not necessarily in a “smooth”manner due to various obstacles, interference, multipath, etc.Similarly, after the receiver passes the signal source, the signalstrength gradually drops off.

The order in which the signal source configuration system 140 observespeak signal values typically corresponds to the order in which thevehicle 370 passes the corresponding signal sources. Thus, the signalsource configuration system 140 can determine based on the signal dataillustrated in FIG. 6 that the peaks occur in the following order: 400A,400B, . . . 400G, that the corresponding signals identify signal sources360A, 360B, . . . 360G, and thus the signal sources are arranged as thesequence 360A, 360B, . . . 360G along the path 352 in the direction 380.

According to one implementation, the signal source configuration system140 further estimates the average distance between the signal sources360A-G using the determined order and by dividing the distance L alongthe path 352 by the number of intervals between the N signal sources:d=L/(N−1). The signal source configuration system 140 can obtain thedistance L using Geographic Information System (GIS) data, map data thatdescribes natural and artificial geographic features provided a mappingservice such as Google Maps, for example, or other suitable geospatialdata. Alternatively, the signal source configuration system 140 mayobtain positioning fixes such as GPS signals from the signal sources360A and 360G, which are positioned outside the area of limitedsatellite coverage, and estimate the distance L using the twopositioning fixes, particularly when the path 352 has low curvature. Thesignal source configuration system 140 then can determine the respectivelocations of these signal sources, using the known location of thesignal source 360A or 360G and/or some other indication of geometry ofthe path 352. More specifically, the signal source configuration system140 can calculate the location of the signal source 360B by applying adisplacement vector of length d to the known location of the signalsource 360A, calculate the location of the signal source 360C byapplying a displacement vector of length d to the known location of thesignal source 360B, etc.

As indicated above, not every pair of adjacent signal sources 360A-G maybe separated by the same distance d, but the signal source configurationsystem 140 may implement this assumption to generate an initialpositioning estimate or when it is known that a constant separation hasbeen maintained during installation of the signal sources 360A-G.

Now referring to FIG. 7, the signal from a signal source 360A-G in somecases may be temporarily obscured by the metal roof of the vehicle inwhich the receiver is disposed. The attenuation can be particularlysignificant for certain frequencies of the signal. Example signal data420 corresponds to one of the signal sources 360A-G. Similar to thesignal data of FIG. 6, the strength of the signal generally increases atthe receiver as the vehicle approaches the signal source and generallydecreases at the receiver as the vehicle moves away from the signalsource. However, here the signal strength reaches a first peak 420,drops off into a “valley” 422, reaches a second peak 424, and graduallydecreases to a low level. The valley 422 likely corresponds to the timewhen the vehicle is below the signal source mounted on the ceiling ofthe tunnel; the peak 420 likely corresponds to the time when the signalfrom the signal source passes through the windshield of the vehicle; andthe peak 424 likely corresponds to the time when the signal passesthrough the rear window of the vehicle. The drop in signal strength inthe valley 422 can be as much as 20 dB, depending on such factors as thefrequency of the signal and the material in the roof of the vehicle.

The signal source configuration system 140 can automatically generate anapproximation 430 for the signal data 420, with a single extrapolatedpeak 432 occurring approximately in the middle of the time intervalbetween the peaks 420 and 424. To this end, the signal sourceconfiguration system 140 can use a suitable statistical technique togenerate one curve for the upward trend in signal strength as thevehicle is approaching the signal source, generate another curve for thedownward trend in signal strength as the vehicle moves away from thesignal source, and identify the peak 432 as an intersection of the twocurves, for example. In view of these trends, the signal sourceconfiguration system 140 can determine that the extrapolated peak shouldbe between the peaks 420 and 424 rather than between the peak 424 andpeak 440, for example.

Additionally or alternatively, the signal source configuration system140 can determine whether a pair of candidate high peaks, such as thepeaks 420 and 424, are separated approximately by the time it takes alength of a vehicle to pass the signal source. For example, assuming thedistance between the point where the receiver in a vehicle “sees” thesignal through the windshield at a certain incident angle and where thereceiver again begins to see the signal through the rear window is 30 m,the peaks 420 and 424 for a vehicle travelling at 100 km/h should beseparated by approximately one second. The signal source configurationsystem 140 accordingly can check whether the time interval betweencandidate peaks appears to be within a reasonable range in view of thespeed of the vehicle (which in turn can be estimated based on thepositioning fixes and the timestamps at the signal sources 360A and360G).

Referring again to FIG. 6, when the receiver in the vehicle 370 obtainspositioning fixes and the corresponding timestamps for the signalsources 360A and 360G, the signal source configuration system 140 candetermine an average speed v_(avg) of the vehicle between signal sources360A and 360G by dividing the distance L by the difference between thetwo timestamps. When the signal sources 360B-F are separated unevenlyrather than with the constant interval of length d, the signal sourceconfiguration system 140 can determine the locations of the signalsources by multiplying the previously determined average speed v_(avg)by the amount of time it takes to reach a certain peak, and calculatingthe distance (or, more generally, a displacement vector) relative to aknown location, such as the entrance into the tunnel.

The signal source configuration system 140 in some implementations alsocan account for variations in speed of the vehicle moving along the path352. Some tunnels are sufficiently long to generate significantvariations in the speed of the vehicle, and traffic or differences inroad quality can result in even greater variations in speed. FIG. 8illustrates highly simplified signal data that a receiver in a vehiclecan collect for the signal sources 360A, 360B, 360C and 360D, forexample. In this scenario, the receiver sees the signal 450A, from thesignal source 360A, above a threshold level 460 for a certain amount oftime t_(A). The threshold level 460 can be set experimentally, but inany case significantly above the noise level to improve accuracy. Inother embodiments, the receiver does not use any particular thresholdvalues and instead analyzes relative changes in signal strength. Thereceiver sees the signals 450C and 450D, from the signal sources 360Cand 360D, for approximately the same amount of time. However, thereceiver sees the signal 450B from the signal source 360B for a longeramount of time t_(B). If the signal sources 360A-G are known to transmitat similar power levels, which is typical, these variations between theamount of time t_(A) and t_(B) are likely to due to differences inspeed.

Assuming the signals 450A-D are transmitted by equally powerfultransmitters, the signal source configuration system 140 can interpretthe signal data illustrated in FIG. 8 as indicating speeds v_(A)-v_(D)for different sections of the path 352, where v_(A) is the speed atwhich the receiver moves during the time t_(A), v_(B) is the speed atwhich the receiver moves during the time t_(B), etc. In this case,v_(A)≈v_(C)≈v_(D), and v_(A)/v_(B)≈t_(B)/t_(A). It is noted that theseestimates do not depend on the actual positioning of the signal sourcesrelative to each other, and only on the duration of the exposure tosignal of a certain strength. Once the respective speed valuesv_(A)-v_(D) have been determined, the signal source configuration system140 can improve the estimate of the locations of the signal sources.

For further clarity, FIG. 9 schematically illustrates how the signalsource configuration system 140 can automatically correct the estimatesof signal source locations in view of varying vehicle speed. Accordingto an initial estimation 520, which may be generated using thetechniques discussed with reference to FIGS. 6 and 7 and the averagespeed of the vehicle, the pairs of signal sources 500/502 and 502/504are separated by distance d, the pairs of signal sources 504/506 and506/508 are separated by a longer distance d′, and the pairs of signalsources 508/510 and 510/512 are separated by a shorter distance d″. Thesignal source configuration system 140 in this scenario takes intoaccount the amount of time during which the receiver in the vehicledetects a signal from a certain signal source at a strength above acertain threshold value. Plot 530 illustrates the duration of exposure,in seconds, of the receiver to a signal source as a function of time.

According to the plot 530, the receiver measures a sufficiently highstrength of the signal from the signal sources 500-504 for approximately12 seconds, from the signal sources 506 and 508 for approximately 18seconds, and from the signal sources 510 and 512 for approximately 8seconds. If the signal sources transmit at approximately the same powerlevel, the plot 530 indicates that the vehicle slowed down near thesignal sources 506 and 508 and sped up near the signal sources 510 and512. Accordingly, instead of calculating a single average speed for theentire path between the signal sources 500 and 512, the signal sourceconfiguration system 140 can determine segment-specific average speedvalues v₁ for the segment corresponding to the signal sources 500-504,v₂ for the segment corresponding to the signal sources 506 and 508, andv₃ for the segment corresponding to the signal sources 510 and 512. Moreparticularly, using timestamp information in the signal data, the signalsource configuration system 140 can determine the approximate durationof time during which the receiver was exposed to the correspondingsignal sources 500-512, and, using positioning fixes, surveying data oranother indication of the endpoints, determine the length L, so that v₁t₁₊ v₂ t₂₊ t₃=L, where t₁ is the amount of time the receiver travelspast the signal sources 500-504 at speed v₁, t₂ the amount of time thereceiver travels past the signal sources 506 and 508 at speed v₂, and t₃is the amount of time the receiver travels past the signal sources 510and 512 at speed v₁.

Upon determining the values v₁,v₂ and v₃, the signal sourceconfiguration system 140 can more accurately determine the locations ofthe signal sources. For example, the signal source configuration system140 can calculate the location of the signal source 504 relative to aknown position P_(i) of the signal source 500 reached at time t_(i),where the observed or extrapolated peak signal strength for the signalsource 504 occurs at time t_(p), by calculating the displacement asd=P_(i)+v₁ (t_(p)−t_(i)). In this example scenario, the signal sourceconfiguration system 140 adjusts the initial estimation 520 to generatean estimation 540, in which all pairs of adjacent signal sources 500-512are separated by distance d. Of course, in other scenarios, the initialestimate can indicate a constant separation d, and an estimate thataccounts for variations in speed indicates different separations d, d′,etc.

The signal source configuration system 140 in some scenarios also canuse speed and/or acceleration data reported by the vehicle in which thereceiver is disposed. For example, a smartphone could access theodometer and other sensor vehicles and report the vehicle sensor dataalong with the signal strength measurements to the signal sourceconfiguration system 140. When this data is available, the signal sourceconfiguration system 140 can improve the accuracy of signal sourceestimation by incorporating additional signal data.

To improve the accuracy of locating signal sources using the techniquesdiscussed above, the signal source configuration system 140 can utilizecrowdsourcing to collect signal data from multiple receivers. The signalsource configuration system 140 then can implement any suitablestatistical approach to apply the techniques discussed above to thesignal data. For example, the signal source configuration system 140 cancombine data from multiple receivers such as by averaging the data (in anon-weighted or weighted manner), eliminating outliers, assigningdifferent weights to the data depending on the level of reliability(e.g., receivers equipped with more robust chipsets may be more accuratewhen measuring signal strength), etc. Further, the signal sourceconfiguration system 140 in some cases can analyze the signal data froma certain receiver to determine whether the receiver operates in aconvertible vehicle or a conventional vehicle, and extrapolate the peaksin signal strength when necessary.

Further, the signal source configuration system 140 in someimplementations can be configured to also account for variations in thecharacteristics of signal sources. Referring for example to FIG. 9, thevariations in signal source visibility in some cases may be due to someof the signal sources 500-512 at a significantly higher power or,conversely, at a significantly lower power due to hardware, battery, orother factors. Further, when the signal sourced 500-512 are beaconsarranged in a tunnel, the physical configuration of the tunnel cancreate variations in how well the signal reaches certain areas. Thesignal source configuration system 140 can collect signal data frommultiple receivers over time to create “profiles” for the individualsignal sources, and use these profiles to further correct the estimatesof the positions of the signal sources.

To generate the profiles, signal data for multiple receivers andmultiple signal sources can be organized into a two-dimensional matrix,with columns corresponding to signal sources and rows corresponding toreceivers, for example. The signal source configuration system 140 canprocess column data to determine the properties of particular signalsources and generate appropriate weights when positioning the signalsources and/or when subsequently positioning receivers. As a morespecific example, the signal sources 500-512 may be implemented asbeacons that transmit indication of transmit power in the managementframes. The signal source configuration system 140 can use the transmitpower indications to calculate correction factors for positioning, asone may expect beacons transmitting at lower power to be “heard” for ashorter period. It has been observed that a 6 dB increase in transmitpower typically results in twice the coverage (e.g., as defined by anarea in which the signal can be received at above a certain fixedthreshold RSSI value), and according in twice the amount of time whenthe beacon is heard.

Further regarding profiling, the characteristics of a certain signalsource can change over time due to obstruction or battery drain, forexample, and the corresponding profile may indicate how the signalsource appears to operate at a particular time. The profile thus mayindicate that signals from a certain source are weakened due to apossible obstructions, so that receivers can adjust the calculationsaccordingly. More generally, the signal source configuration system 140can determine one or multiple characteristics of a signal source toenable adjustments during geopositioning.

Although the examples above were discussed with specific reference tothe signal sources 500-512 of FIG. 9 or 360A-G of FIG. 5, it will beunderstood that in general each of these techniques can apply to varioussuitable signal sources, which may include the example signal sources ofFIG. 1, 5, or 9.

Now referring to FIG. 10, an example method 550 for determininggeographic positions of signal sources in areas with limited satellitecoverage can be implemented in the signal source configuration system140 or another suitable system. The method 550 can be implemented as aset of instructions in any one or several suitable programminglanguages, stored on a non-transitory computer-readable medium andexecutable by one or more processors.

The method 550 begins at block 552, where signal data is obtained fromone or more receivers moving along a path in an area with limitedsatellite coverage, e.g., on a road through a tunnel. The signal datacan be indicative to changes in signal strength over time, as discussedabove, and in some implementations can include such information assignal source identifiers and transmit power setting at the signalsource. Moreover, in some implementations, once the locations of signalsources have been determined, the positioning information can betransmitted to the corresponding signal sources for subsequenttransmission.

Next, the relative order of signal sources arranged along path isdetermined using the signal data at block 554. To this end, peak valuesfor different signal sources can be identified, where the relativetiming of the peaks corresponds to the order in which the receiverapproaches the signal sources, as discussed above with reference toFIGS. 6-8. At block 556, positioning fixes and timestamps for theendpoints of the path can be received. Signal sources positioned atentrances to the tunnels can be manually provisioned with a GPSpositioner, or can be placed at known surveyed locations, or can obtainGPS positioning fixes in real time, for example.

At block 558, the determined relative order of the signal sources can beused for approximate positioning, permanently or temporarily whileadditional information is collected. For example, if the assumption thatthe signal sources are spaced apart at regular intervals is implemented,the approximate locations of the signal sources can be determined usingthe locations of the endpoints to determine the distance between theendpoints and dividing the distance by the number of intermediate signalsources to obtain a displacement relative to known locations. Theestimates of the locations of the signal sources can be further improvedusing vehicle speed, profiling of individual signal sources, and othertechniques discussed above.

At block 560, the determined locations of the signal sources can be usedto geoposition a receiver subsequently moving along the same path pastthe signal sources. Continuous improvement of the estimation of signalsource locations also can occur in parallel with positioning ofreceivers using the currently available estimates. For example, avehicle moving through a tunnel can request positioning using theavailable beacons while also contributing its signal strengthmeasurements to the set of signal data used in signal source locationdetermination.

Automatic Provisioning of Access Points Using RTT Measurements

Now referring to FIG. 11, example stations 600A-E operate as bothtransmitters and receivers and are configured to automatically determinetheir locations using round trip-time (RTT) measurements. These stationsexchange management frames such as beacon frames, for example, andmeasure RTT to determine distances d₁-d₄. As an alternative to RTTmeasurements, the stations 600A-E can use other suitable metrics usingwhich the distance between a pair of stations can be determined. Forexample, the stations 600A-E can use known transmit power, RSSImeasurements, and models that predict the loss in RSSI over distance toestimate distances to other stations.

The corresponding ranges of the signals transmitted by the stations600A-E are schematically represented by circles 610A-E. The radii of thecircles 610A-E typically are similar, but need not be the same for thepurposes of the techniques of this disclosure. As illustrated in FIG.11, the station 600A can “hear” the station 600B, the station 600B canhear the stations 600A and 600C, the station 600C can hear the stations600B, 600D, and 600E, the station 600D can hear the stations 600B, 600C,and 600E, and the station 600E can hear the stations 600C and 600D. Thestations 600A-E can use the one, two, or more signals from the otherstations to estimate the distance to one or more neighboring stations.Moreover, when the arrangement of the stations 600A-E along the path canbe described using one-dimensional geometry (i.e., at differentpositions at substantially the same distance from the path, such as onthe ceiling of a tunnel), the stations 600A-E also can use theindication of geometry to determine their positions.

In other embodiments, the arrangement of the stations 600A-E can bedescribed using two-dimensional or three-dimensional geometry. Forexample, a station can determine both the position along a line, acurve, a polyline, etc. on a plane representing a road and the height atwhich this or another station is disposed. Moreover, the stations 600A-Emay not be arranged along a road with known geometry when used in aparking garage, a mall, or another area in which receivers can move innumerous directions. The station in these cases may use more signalsfrom the other stations, when available, to determine positions.

For clarity, FIG. 12 illustrates a flow diagram of an example method 650for provisioning the stations 600A-E or other signal sources capable ofalso receiving wireless signals. The method 650 can be implemented inthe station 600A-E as a set of instructions stored on a non-transitorycomputer-readable medium, such as flash memory, of the station, andexecuted by one or more processors of the station. In some alternativeimplementations, the method 650 at least partially is implemented in anetwork device or system such as the signal source configuration system140.

The method 650 begins at block 652, where an indication of geometry of apath traversing an area with limited satellite coverage is received. Thegeometry can be simple (e.g., a straight line, an arc) or relativelycomplex (e.g., a polyline). As discussed above, the path typically is aroad through a tunnel. However, the method 650 also can be used withareas in which there is a large number of possible paths a receiver cantraverse, and where the receiver has more degrees of freedom that avehicle moving along a road, for example. Examples of such areas includeparking garages and shopping malls. The description of geometry in thesecases can be a polygon enclosing an area or even a set of verticesdescribing a three-dimensional volume.

At block 654, signal data indicative of distances between the stationsis received. The signal data can be generated using management frames ofany suitable type which the stations can receive from each other. Theframes may include identifiers of the corresponding stations.

The locations of the stations then are determined using the indicationof the geometry and the signal data, at block 656. Referring back toFIG. 11, the station 600A can determine the distance to the station 600Busing the signal from the station 600, and the station 600B candetermine the distance to the station 600A using a signal from thestation 600A and determine the distance to the station 600C using asignal from the station 600C. When additional information is available,such as the signals from both the station 600D and 600E at the station600C, the data can be averaged to arrive at a more accurate estimate.Further, similar to the examples discussed above, determining stationlocations can include using a known position of a station positioned atthe entrance to an area with limited satellite coverage.

At block 658, the determined locations can be used to geoposition adevice moving along the same path. It is noted that the techniquesdiscussed with reference to FIGS. 10 and 11 allow the stations todetermine at least positions relative to the neighboring stationswithout relying on a network server. Of course, the techniques discussedwith reference to FIGS. 10 and 11 also can be used with a network serveror a group of servers, if desired. In some embodiments, the techniquesof FIGS. 10 and 11 are used with signal sources that utilize widebandwidth to reduce multi-path problems.

Additional Considerations

The various operations of example methods described herein may beperformed, at least partially, by one or more processors that aretemporarily configured (e.g., by software) or permanently configured toperform the relevant operations. Whether temporarily or permanentlyconfigured, such processors may constitute processor-implemented modulesthat operate to perform one or more operations or functions. The modulesreferred to herein may, in some example embodiments, compriseprocessor-implemented modules.

Similarly, the methods or routines described herein may be at leastpartially processor-implemented. For example, at least some of theoperations of a method may be performed by one or more processors orprocessor-implemented hardware modules. The performance of certain ofthe operations may be distributed among the one or more processors, notonly residing within a single machine, but deployed across a number ofmachines. In some example embodiments, the processor or processors maybe located in a single location (e.g., within a home environment, anoffice environment or as a server farm), while in other embodiments theprocessors may be distributed across a number of locations.

The one or more processors may also operate to support performance ofthe relevant operations in a cloud computing environment or as asoftware as a service (SaaS). For example, at least some of theoperations may be performed by a group of computers (as examples ofmachines including processors), these operations being accessible via anetwork (e.g., the Internet) and via one or more appropriate interfaces(e.g., application program interfaces (APIs).)

The performance of certain of the operations may be distributed amongthe one or more processors, not only residing within a single machine,but deployed across a number of machines. In some example embodiments,the one or more processors or processor-implemented modules may belocated in a single geographic location (e.g., within a homeenvironment, an office environment, or a server farm). In other exampleembodiments, the one or more processors or processor-implemented modulesmay be distributed across a number of geographic locations.

What is claimed is:
 1. A system for geopositioning receivers in areaswith limited satellite signal coverage, the system comprising: aplurality of access points configured to periodically transmitmanagement frames (i) via a same shared communication channel and (ii)using a modulation scheme that provides a rate of at least 50 Mbps; adatabase that stores respective locations for each of the plurality ofaccess points; a portable computing device configured to: retrieve thelocations of the plurality of access points from the database, and whenmoving through a region in which the plurality of access points aredisposed, (i) receive at least one management frame from each of theplurality of access points on the shared communication channel, within ascanning interval corresponding to a scan of the shared communicationchannel during a multi-channel scan procedure, prior to switching toanother communication channel, each channel corresponding to a differentfrequency or frequency band, and (ii) determine a current position ofthe portable computing device based on the management frames using theretrieved locations of the plurality of access points.
 2. The system ofclaim 1, wherein the portable computing device is further configured toretrieve channel selection information from the database.
 3. The systemof claim 1, wherein the plurality of access points are disposed within aregion of limited satellite coverage, and wherein the portable device isconfigured to retrieve the locations of the plurality of access pointsprior to entering the region.
 4. The system of claim 1, wherein themodulation scheme is OFDM.
 5. The system of claim 1, wherein theplurality of access points are configured according to one of IEEE802.11 or IEEE 802.15 standard.
 6. The system of claim 1, wherein the atleast one management frame includes a beacon.
 7. A method forgeopositioning receivers in areas with limited satellite signalcoverage, the method comprising: configuring, by one or more processors,a plurality of access points to periodically transmit management frames(i) via a same shared communication channel and (ii) using a modulationscheme that provides a rate of at least 50 Mbps; providing, by one ormore processors, respective locations for each of the plurality ofaccess points to a portable computing device; wherein the portablecomputing device, when moving through a region in which the plurality ofaccess points are disposed, is configured to receive at least onemanagement frame from each of the plurality of access points on theshared communication channel, within a scanning interval correspondingto a scan of the shared communication channel during a multi-channelscan procedure, prior to switching to another communication channel,each channel corresponding to a different frequency or frequency band,and determine a current position of the portable computing device basedon the management frames using the retrieved locations of the pluralityof access points.
 8. The method of claim 7, wherein the plurality ofaccess points are disposed within a region of limited satellitecoverage, and wherein the portable device is configured to retrieve thelocations of the plurality of access points prior to entering theregion.
 9. The method of claim 7, wherein the modulation scheme is OFDM.10. The method of claim 7, wherein the plurality of access points areconfigured according to one of IEEE 802.11 or IEEE 802.15 standard. 11.The method of claim 7, wherein the at least one management frameincludes a beacon.
 12. A portable computing device comprising: one ormore processors; a non-transitory computer-readable medium storinginstructions that, when executed by the one or more processors, causethe portable computing device to: receive at least one management framefrom each of a plurality of access points disposed in a region withlimited satellite signal coverage, including receiving the managementframes via a same shared communication channel, prior to switching toanother communication channel, each channel corresponding to a differentfrequency or frequency band, and using a modulation scheme that providesa rate of at least 50 Mbps, within a scanning interval corresponding toa scan of the shared communication channel during a multi-channel scanprocedure, determine respective locations of the plurality of accesspoints, and determine a current position of the portable computingdevice based on the management frames using the retrieved locations ofthe plurality of access points.
 13. The portable computing device 12,wherein the instructions cause the portable computing device to retrievethe respective locations of the plurality of access points from adatabase via a communication network.
 14. The portable computing device12, wherein the instructions cause the portable computing device toretrieve the respective locations from the received management frames.15. The portable computing device 12, wherein the instructions cause theportable computing device to retrieve the locations of the plurality ofaccess points prior to entering the region with limited satellite signalcoverage.
 16. The portable computing device 12, wherein the modulationscheme is OFDM.
 17. The portable computing device 12, wherein the atleast one management frame includes a beacon.